# Mass Transport: Circulatory System
Large organisms cannot rely on diffusion alone. The circulatory system provides mass transport of oxygen, nutrients, hormones, and waste products.
1. The Heart
- Double circulatory system: pulmonary + systemic
- Four chambers: right atrium → right ventricle → lungs → left atrium → left ventricle → body
- Left ventricle: thickest wall (pumps to body)
- Coronary arteries supply heart muscle
Cardiac Cycle
- Atrial systole: atria contract → blood pushed into ventricles through AV valves
- Ventricular systole: ventricles contract → blood pushed into arteries through semilunar valves; AV valves close
- Diastole: all chambers relax → blood fills atria from veins
Pressure changes drive valve opening/closing.
2. Heart Rate Control
- SAN (sinoatrial node): pacemaker in right atrium → generates electrical impulse
- AVN (atrioventricular node): delays impulse → atria finish contracting before ventricles start
- Bundle of His → Purkinje fibres: conduct impulse through ventricle walls (from apex up)
Autonomic nervous system: sympathetic (speeds up), parasympathetic/vagus nerve (slows down)
3. Blood Vessels
| Feature | Arteries | Capillaries | Veins |
|---|---|---|---|
| Wall | Thick, muscular, elastic | One cell thick | Thin, less muscle |
| Lumen | Small | Very small | Large |
| Blood pressure | High | Low | Very low |
| Valves | No (elastic recoil) | No | Yes (prevent backflow) |
| Function | Carry blood from heart | Exchange substances | Return blood to heart |
4. Haemoglobin and Oxygen Transport
Haemoglobin (Hb): quaternary protein; 4 subunits; each with haem group (contains Fe²⁺).
Oxygen Dissociation Curve
- Sigmoid (S-shaped) due to cooperative binding
- At lungs (high pO₂): Hb binds O₂ readily
- At tissues (low pO₂): Hb releases O₂
Bohr Effect
- Increased CO₂/decreased pH → curve shifts RIGHT
- Hb has lower affinity for O₂ → more O₂ released to active tissues
Fetal Haemoglobin
- Higher affinity for O₂ than maternal Hb
- Curve shifted LEFT → fetus can take O₂ from mother's blood
5. Tissue Fluid and Lymph
- Arterial end: high hydrostatic pressure → fluid forced out of capillaries (ultrafiltration)
- Venous end: lower hydrostatic pressure; oncotic pressure (from plasma proteins) draws fluid back
- Excess tissue fluid drains into lymph capillaries → lymphatic system → returned to blood
6. Practice Questions
- Describe the cardiac cycle with reference to pressure changes.
- Explain the Bohr effect and its significance.
- Why is fetal haemoglobin's dissociation curve to the left of adult?
- Describe how tissue fluid is formed.
- Explain the role of the SAN and AVN in heart rate control.
Want to check your answers and get step-by-step solutions?
Summary
- Cardiac cycle: atrial systole → ventricular systole → diastole
- SAN → AVN → Bundle of His → Purkinje fibres
- Haemoglobin: cooperative binding; sigmoid curve
- Bohr effect: CO₂ shifts curve right → more O₂ released
- Tissue fluid: formed by ultrafiltration; returned by osmosis and lymph